In large industrial plants such as petroleum refineries, pneumatic tube systems are sometimes used to transport small samples of plant streams or tanks to the laboratory for analysis. This system saves a significant amount of time over that required for reqular scheduled sample pickup by truck. The sample, usually a liquid, is poured into a plastic bottle and the bottle is capped. The capped bottle is placed in a metal cylinder or carrier, and the carrier is then inserted in the pneumatic tube for transportation to the laboratory. These systems may operate under pressure or vacuum, although vacuum systems are more commonly used. In vacuum systems, a pump is used to draw air from the remote end of the tube to the end at the laboratory. Pressure drop in the tube results in a partial vacuum in the line. The flow of air through the tube propels the carrier to the laboratory at the end of the tube. Typical systems use tubes from four to six inches in diameter, and may be from a few hundred feet to over a mile in length.
The longer systems require several pumps to provide a more even air flow in the tube and to overcome the greater frictional effect of the longer tube. These multipump systems are divided into sections by normally closed valves to prevent two pumps from working against one another. Each pump has its own section of tubing. The valves may be motor operated slide valves or spring loaded flapper valves. Carriers move through the tube typically at speeds of from thirty to forty feet per second. In the case of the slide valve, when the carrier approaches the valve, it trips an electrical switch which activates a motor to open the valve, allowing the carrier to pass through. The valve then closes automatically. This valve system is relatively complicated, expensive, and requires more maintenance than the spring loaded valve. When spring loaded valves are used, the valve is held shut by a combination of pressures from the spring and the vacuum in the tube. The moving carrier strikes the valve flap with a very high impact force and kicks it open, being carried past the valve flap by its momentum. The valve then closes automatically.
Experience gained in a large petroleum refinery from several years operation of a pneumatic tube system employing flapper valves indicates that proper valve design is an important factor in obtaining reliable operation. Commercially available flapper valves are supplied with a single springloaded flap which is faced with leather or rigid laminated canvas. These valves have been unsatisfactory for several reasons, and their use has lead to excessive mechanical failure and consequent excessive blockage of the pneumatic tube. When a carrier, which typically weighs several pounds, strikes a flap seal while traveling at a speed of from 30 to 40 feet per second, the flap seal is kicked open with a tremendous impact force. This great impact force results in metal fatigue and rather rapid wearing of both the valve and the carrier with consequent frequent malfunctioning of these commercially available valves. Further, the single flap design forces the carrier to one side of the valve which results in additional valve and carrier wear. Also, air intake in the commercially available flapper valves is often directly from the surrounding air. Thus moisture can be drawn into the line from the atmosphere in the form of rain, snow or steam. At subfreezing temperatures ice is formed and tube blockage sometimes results.